Can I apply with a 3 year or a 4 year bachelor's?

Short answer: A 4 year honours degree is the safest. A 3 year degree is not automatically a problem, because Italy's own bachelor's is 3 years. What matters is whether your degree is an official bachelor's that lets you study a master's in your own country.

What Italy checks (MUR circular)

An official first cycle degree (European level 6) from an accredited institution.
It allows you to start a master's in your own country.
No "substantial difference" compared with an Italian bachelor's.

What this means in practice

A 4 year honours degree from Bangladesh fits.
A 3 year degree (pass course) is judged case by case. Email the admissions office with your transcript before you apply or pay anything.
Whatever your degree, send the full transcript. The university decides, not a rule of thumb.

This page explains official rules in plain language. I am not an immigration adviser and I do not give visa advice. For that, always use official government sources. Rules change: confirm with your university, the embassy and VFS before you act.